Maybe a solution would be to not allow any trading of Tech Theories ('golds') and limited trading of Applications ('grays), perhaps 1 trade quarterly. This would hopefully slow unbalancing tech trades, while allowing some historical situations (the American P-51 Mustang was a low-level multirole fighter until the Brits gave them the design for the Merlin engine)
(Commonwealth forces in Europe used American tanks with British guns-Sherman Firefly)
(Not sure if it's a coincidence, but the Me262 and J8M Shusui look a lot alike. I think there's a twin of the Me163 rocket fighter, but I'm not sure)
(Japanese soldiers in Battlefield 1942 use Kar98 rifles and MP43 assault rifles

)
Point is, it's fun in a 'what-if' kind of way to give minor tech away, and if done sparingly, shouldn't unbalance things much. The 'golds', however, represent leading-edge ideas and I would think that they should be kept secret. I think the game is setup so that each team has a more or less equal chance of winning (remember, winning means staying alive and your alliance having more points than any other). So Italy doesn't need to conquer South America to win the game; just stay alive and contribute to an Axis win.
Each side has certain advantages and disadvantages, so when you have unlimited trading, you change the designed odds for victory, which are roughly even, to heavily favor the side with the most countries researching tech.
Having said that, If you want to have a game where anyone can do anything, that could be fun too, as countries do stuff their historical counterparts never dreamed of. But if you have unlimited tech trading, as soon as the alliances are formed, you will know who is going to win. Not my cup of tea. (although I would still have SOME fun)
